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checkout.amount_out_of_rangecheckoutRule C2The amount is outside the payment link's range
An open-amount payment link sets a minimum and a maximum, and the amount the payer submitted is outside it. A fixed-amount link takes no amount at all — it uses its own.
Why the API refuses
WHEN a payment link with a fixed amount is opened, THE SYSTEM SHALL create the payment with the link's amount, description and metadata; IF the link has an open amount THEN THE SYSTEM SHALL require an amount within min and max and reject anything outside it.
What to do
Read the link before rendering its form: a fixed link means no amount input, an open one means an input bounded by min and max. Validate on your side too, so a payer finds out before they submit rather than after.